To focus an image on an imaging array in a conventional digital camera a lens adjustment assembly is used to mechanically move one or more lenses in a linear direction relative to an image array from a known starting position, such as “infinity”, to a focused position, at which the image is focused on the image array. The lens is then held at the focused position, while the image is acquired. Many lens adjustment assemblies move the lens using an electromechanical actuator, which may include technology such as voice-coils, electrically active polymers, or piezoelectric actuators. This movement may be made against resistance from a bias source, for example, a spring, which serves to return the lens to the lens starting position when the actuator is turned off.
Most cameras, including digital cameras, have an automatic focus feature (referred to herein as “auto focus”) by which objects viewed through the camera can be focused on automatically. Auto focus systems are generally categorized as either active or passive systems. Active systems actually determine the distance between the camera and the subject of the scene, e.g., by measuring the total travel time of ultrasonic waves or infrared light emitted from the camera. Based on the total travel time, the distance between the camera and the subject of the scene may be calculated and the lens adjustment assembly moves the lens from the starting position to a focused position correlated to the calculated distance of the camera to the object.
Passive auto focus systems, on the other hand, rely on the light that is naturally reflected by the subject in the scene. One example of a passive auto focus system is a system that uses contrast analysis of a captured image to determine the best focal position for the camera lens. In a contrast analysis auto focus system, adjacent areas of a scene are compared with each other to measure differences in intensity among the adjacent areas. An out-of-focus scene will include adjacent areas that have similar intensities, while a focused scene will likely show a significant contrast between areas in which the subject of the scene is located and other areas of the scene (e.g., background objects). During focusing, the lens adjustment assembly moves the lens from the starting position to a number of intermediate positions until the focused position is finally determined (that is, when the lens position results in an image having the maximum intensity difference between adjacent areas).
In either passive or active auto focus systems, energy is used by the electromagnetic actuator to move and maintain the lens at various positions against the resistance of the spring. FIG. 1 shows the relative amount of power needed for the distance that the lens is to be displaced in a conventional lens adjustment assembly. It can be seen from FIG. 1 that the power needed to displace the lens increases as the amount of displacement increases. In some conventional lens adjustment assemblies, the electromagnetic actuators may require a large amount of power, which, for imaging devices operating on a limited power supply such as batteries, will drain the batteries and diminish the usefulness of the imaging device.
Accordingly, there is a desire and need for an imaging device with an auto focus capability that mitigates against these shortcomings.